The continuously increasing requirements for thin film magnetic recording media with very high areal recording densities impose increasingly greater requirements on the magnetic properties of the various thin film layers constituting the media, such as increased remanent magnetic coercivity (Hcr), and coercivity squareness (Sr*), low medium noise, e.g., expressed as signal-to-medium noise ratio (SMNR), and improved narrow track recording performance. As the areal recording density requirement increases, it becomes increasingly difficult to fabricate magnetic recording media, e.g., thin film longitudinal media, which satisfy each of these demanding requirements.
The ever-increasing demands for magnetic recording media with higher storage capacity, lower noise, and reduced cost have precipitated research with the aim of reducing the size of the magnetic grains necessary to record bits of information, while maintaining the integrity of the information as the grain size is progressively reduced. The area or space necessary to record information in magnetic recording media depends upon the size of the transitions between oppositely magnetized areas or domains. It is, therefore, desirable to form magnetic media which support the smallest possible transition size. However, the signal output from media with very small transition sizes must avoid excessive noise to reliably maintain the integrity of the stored information. Media noise is generally characterized as the sharpness of a signal on read-back, against the sharpness of a signal on writing of the media and is generally expressed as the signal-to-medium noise ratio (SMNR).
The linear recording density can be increased by increasing the H, of the media, however, this objective can only be achieved by decreasing the media noise, as by formation and maintenance of magnetic recording layers with very finely dimensioned, non-magnetically coupled grains. Media noise is a dominant factor restricting obtainment of further increases in areal recording density of high density magnetic hard disk drives. The problem, or cause, of media noise is generally attributed, in large part, to inhomogeneous magnetic grain size and inter-granular exchange coupling. Accordingly, it is considered that, in order to increase linear recording density of thin film magnetic media, the media noise must be minimized by suitable control of the microstructure of the magnetic recording layer(s).
A portion of a conventional thin film, longitudinal magnetic recording medium 1, such as is commonly employed in hard disk form in computer-related applications, is depicted in FIG. 1 in simplified, schematic cross-sectional view, and comprises a substantially rigid, non-magnetic substrate 10, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Al—Mg) alloy, or of glass, glass-ceramic, etc., having sequentially deposited or otherwise formed on a surface 10A thereof a plurality of thin film layers. When substrate 10 comprises Al or an Al-based alloy a plating layer 11, such as of amorphous nickel-phosphorus (Ni—P), is typically initially provided on substrate surface 10A (such NiP plating layer 11 generally is omitted when substrate 10 comprises glass). The plurality of thin film layers formed over plating layer 11 or substrate surface 10A include a system 12 of layers for control of the microstructure of medium 1, comprising a first, or seed layer 12A of an amorphous or fine-grained material, e.g., a chromium-titanium (Cr—Ti) alloy and a second, polycrystalline underlayer 12B, typically of Cr, a Cr-based alloy, or other BCC-structured alloy; an intermediate (or “onset”) layer 13 of a magnetic alloy, such as a Co-based alloy, e.g., CoCrTa, CoCrPtNi, CoCrTaPt, CoCrPtB, and CoCrTaBPt, for providing further improvement of the microstructure, texture, and crystallographic orientation of overlying magnetic recording layer(s); at least one magnetic recording layer 14, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 15, typically containing carbon (C), e.g., diamond-like carbon (“DLC”); and a lubricant topcoat layer 16, e.g., of a perfluoropolyether. Each of layers 11-15 may be deposited by suitable physical vapor deposition (“PVD”) techniques, such as sputtering, and layer 16 is typically deposited by dipping or spraying.
In operation of medium 1, the at least one magnetic layer 14 is locally magnetized by a write transducer, or write “head”, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coereivity of the material of the recording medium layer 13, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the at least one recording layer 14 can subsequently produce an electrical response in a read transducer, or read “head”, allowing the stored information to be read.
As indicated above, it is recognized that the magnetic properties which are critical to the performance of the at least one magnetic recording layer 14, i.e., coercivity Hcr, magnetic remanence-thickness product Mrt, coercivity squareness S*, signal-to-media noise ratio SMNR, and thermal stability characteristics, depend primarily on the microstructure of the at least one magnetic recording layer 14 which, in turn, is strongly influenced by the microstructure of the underlying system 12 of seed and underlayers 12A and 12B, respectively. It is also recognized that underlayers having a very fine grain structure are highly desirable, particularly for growing fine grains of hexagonal close-packed (hcp) Co-based magnetic alloys deposited thereon.
Notwithstanding improvements made in recent years in the performance of high areal density longitudinal thin-film magnetic recording media, e.g., as by interposition of intermediate magnetic layer 13 between the at least one magnetic recording layer 14 and underlayer 12B, as described supra, further improvement in the performance of high areal density thin film longitudinal recording media is desired.
Accordingly, there exists a need for improved high areal density, thin film longitudinal magnetic recording media exhibiting enhanced bulk magnetic properties, e.g., thermal stability, increased coercivity Hcr, and increased squareness S*, without adverse effects on the magnetic remanence-thickness product Mrt.
The present invention, therefore, addresses and solves problems-attendant upon the manufacture of thermally stable, high areal density thin film longitudinal magnetic recording media, while affording full compatibility with all technical and economic aspects of conventional automated manufacturing technology for fabrication of thin film magnetic recording media.